JPWO2017042862A1 - Solar cell manufacturing method and solar cell - Google Patents

Abstract

In order to obtain a method for manufacturing a solar cell having a long carrier life by suppressing the contamination of impurities on the back surface when performing impurity diffusion by heat treatment after film formation of the solid phase diffusion source, the light receiving surface 1A and A step of forming a BSG film 2 as a solid phase diffusion source on the light-receiving surface 1A of the n-type single crystal silicon substrate 1 having the back surface 1B, and heating the n-type single crystal silicon substrate 1 from the BSG film 2 A boron-containing product 4 formed on the back surface 1B prior to the heat treatment step, a silicon oxide-containing product 5, and the like. Removing the solid phase diffusion source.

Description

  The present invention relates to a method for manufacturing a solar cell and a solar cell, and particularly relates to an improvement in photoelectric conversion efficiency.

  Conventionally, in a solar cell, as shown in Patent Document 1, for example, a diffusion source is formed using a CVD method or the like as a method for diffusing impurities to a light receiving surface that is a light incident surface or a back surface that is opposite to the light receiving surface. A method is disclosed in which after the film is formed, the substrate and the film serving as a diffusion source are heated in a nitrogen atmosphere to diffuse impurities into the substrate.

JP 2004-247364 A

  However, in the method for manufacturing a solar cell disclosed in Patent Document 1, a phosphorous silicate glass (PSG) film or a boron silicate glass (BSG) film is formed on a substrate. Heat treatment for impurity diffusion is performed in a nitrogen atmosphere later. For this reason, there is a problem in that impurities such as phosphorus or boron circulate on the back surface of the substrate during film formation, and impurities diffuse to the back surface simultaneously from the attached product, so that impurities are unintentionally mixed into the back surface. Impurity contamination leads to a reduction in the carrier life of the solar cell.

  The present invention has been made in view of the above, and after forming a solid phase diffusion source, when impurities are diffused by heat treatment, impurities are prevented from being mixed into the back surface, so that a solar with a long carrier life can be obtained. The purpose is to obtain a battery.

  In order to solve the above-described problems and achieve the object, the present invention provides a process for forming a solid phase diffusion source on a first main surface of a first conductivity type semiconductor substrate having first and second main surfaces. And a heat treatment step of heating the semiconductor substrate, diffusing impurities of the second conductivity type from the solid phase diffusion source, and forming a diffusion layer of the second conductivity type, prior to the heat treatment step And a step of removing the solid phase diffusion source formed on the second main surface.

  According to the present invention, after the solid phase diffusion source is formed, when impurities are diffused by heat treatment, it is possible to prevent impurities from being mixed into the back surface and improve the carrier life of the solar cell. To do.

1 is a flowchart showing a method for manufacturing a solar cell according to a first embodiment. (A) to (d) is a process cross-sectional view illustrating the method for manufacturing the solar cell according to the first embodiment. (A) to (d) is a process cross-sectional view illustrating the method for manufacturing the solar cell according to the first embodiment. Explanatory drawing which shows the time chart about the temperature in a furnace and environmental state of the heat treatment process in the manufacturing process of the solar cell concerning Embodiment 1. FIG. (A) And (b) is a figure which shows the cross section of an n-type single crystal silicon substrate when the film-forming defect part arises partially at the time of formation of a BSG film and a silicon oxide film in the method of Embodiment 1. A flowchart which shows the manufacturing method of the solar cell concerning Embodiment 2. FIG. Process sectional drawing which shows the principal part of the manufacturing process of the solar cell concerning Embodiment 2 The figure which shows an example of the injection | throwing-in method of the silicon substrate to the diffusion furnace used at the time of a heat processing with respect to the manufacturing method of the solar cell shown in Embodiment 1 and Embodiment 2

  Below, the manufacturing method of the solar cell concerning this invention and embodiment of a solar cell are described in detail based on drawing. In addition, this invention is not limited by this embodiment, In the range which does not deviate from the summary, it can change suitably. In the drawings shown below, the scale of each layer or each member may be different from the actual for easy understanding, and the same applies to the drawings. Further, even a plan view may be hatched to make the drawing easy to see.

Embodiment 1 FIG.
FIG. 1 is a flowchart of a manufacturing process showing Embodiment 1 of a method for manufacturing a solar cell according to the present invention. FIGS. 2 (a) to (d) and FIGS. 3 (a) to (d) FIG. 10 is a process cross-sectional view illustrating the solar cell manufacturing method according to the first embodiment. 2 (a) to 2 (d) are cross-sectional views showing changes in the solar cell substrate in the continuous processing in the furnace shown in FIG. 1 in the method for manufacturing a solar cell according to the present invention. 3 (a) to 3 (d) are schematic views showing changes in the cross section of the solar cell in the process following the heat treatment shown in FIGS. 2 (a) to 2 (d) during the manufacturing process of the first embodiment. . FIG. 4 is an explanatory diagram showing a time chart regarding the temperature in the furnace and the environmental state.

  The solar cell manufacturing method according to the first embodiment includes a step of removing the solid phase diffusion source formed on the second main surface prior to the heat treatment step in the heat treatment step for forming the diffusion layer. It is a feature.

  In other words, the present embodiment includes a step of removing the solid phase diffusion source formed on the second main surface prior to the step of diffusing impurities from the solid phase diffusion source by the heating step. The solid-phase diffusion material wraps around and adheres to the surface opposite to the surface on which the source is deposited. By removing the solid-phase diffusion source and then performing heat treatment, impurities from the deposit are diffused into the substrate. This is intended to avoid this.

  The solar cell according to the first embodiment uses an n-type single crystal silicon substrate 1 as a first conductivity type semiconductor substrate having a first main surface serving as a light receiving surface 1A and a second main surface serving as a back surface 1B. . The manufacturing method will be described with reference to FIGS. 1, 2A to 2D, FIGS. 3A to 3D, and FIG. First, in the damaged layer removing step S101, contamination or damage caused during wafer slicing on the surface is removed by immersing in an alkaline solution in which, for example, 1 wt% or more and less than 10 wt% sodium hydroxide is dissolved, and then the n-type single layer is removed. An unevenness for obtaining an antireflection structure by adding an additive such as isopropyl alcohol or caprylic acid in an alkaline solution of 0.1% or more and less than 10% to the light receiving surface 1A of the crystalline silicon substrate 1, for example. A certain texture is formed. The removal of slice contamination and damage and the formation of texture may be performed simultaneously or individually. The texture may be formed not only on the light receiving surface but also on the back surface. 2 and 3, the texture is not shown for easy understanding, and both the light receiving surface and the back surface are shown as flat surfaces.

  Next, the surface of the n-type single crystal silicon substrate 1 is cleaned in a pre-deposition cleaning step S102. In the cleaning step, for example, a mixed solution of sulfuric acid and hydrogen peroxide, a hydrofluoric acid aqueous solution, a mixed solution of ammonia and hydrogen peroxide, a mixed solution of hydrochloric acid and hydrogen peroxide, called RCA cleaning, A step of removing an organic material, a metal, and an oxide film combined with each other, or a step of removing an oxide film using only a hydrofluoric acid aqueous solution is used depending on, for example, a texture forming method. As for the cleaning solution, select one or more of the types of cleaning solutions, or select a cleaning solution including a mixed solution of hydrofluoric acid and hydrogen peroxide, or water containing ozone. Also good.

  In addition, in order to ensure that each processing solution itself does not cause contamination to others or unintended reactions, and to ensure safety after removal from the equipment, it is necessary to use pure water at each intermediate or pre-drying stage. Wash with water.

  Subsequent to the cleaning step, in the film forming step S103 on the light receiving surface 1A side of the solid phase diffusion source, as shown in FIG. For example, a boron silicate glass (BSG) film 2 which is an oxide film containing boron is formed. For film formation, for example, low pressure CVD (Chemical Vapor Deposition) or atmospheric pressure CVD is used. Note that the boron-containing product 4 adheres to the back surface 1B of the n-type single crystal silicon substrate 1 by the wrapping of the film forming gas during the film forming process described above. Subsequently, a film that becomes a cap at the time of heat treatment, for example, a silicon oxide film 3 is formed on the BSG film 2. The silicon oxide film 3 is preferably formed by a film forming process such as low pressure CVD or atmospheric pressure CVD like the BSG film 2 in view of the continuity of the process. When the silicon oxide film 3 is formed, as in the case of forming the BSG film 2, the film forming gas flows around the back surface 1B and the silicon oxide-containing product 5 adheres.

  In the back surface solid phase diffusion source removal step S104, as shown in FIG. 2B, the back surface 1B side solid phase diffusion source is removed. Here, the solid phase diffusion source on the back surface 1B side of the n-type single crystal silicon substrate 1 is removed. That is, after the formation of the BSG film 2 and the silicon oxide film 3, the boron-containing product 4 and the silicon oxide-containing product 5 on the back surface 1B side are removed. The removal is performed, for example, by dissolution using a hydrofluoric acid aqueous solution. However, since the boron-containing product 4 is essentially the same material as the BSG film 2 and the silicon oxide-containing product 5 is, for example, single-sided etching. It is desirable to remove the boron-containing product 4 and the silicon oxide-containing product 5 by bringing the hydrofluoric acid aqueous solution into contact with only the back surface 1B side using an apparatus. As an example of a single-sided etching apparatus, single-sided etching can be realized by using an apparatus in which an etching solution is sprayed from below with an etching surface down or an etching device having a structure in which only one surface is immersed in an etching solution.

  The n-type single crystal silicon substrate 1 is subjected to heat treatment continuously. A heat treatment furnace is used for the heat treatment. First, in step S105 in which the heat treatment furnace is preheated and heat treatment is performed in an inert gas atmosphere, as shown in FIG. 2C, heat treatment is performed in an inert gas atmosphere to form a diffusion layer.

Next, in step S106 in which heat treatment is continuously performed in an atmosphere containing oxygen O ₂ , the heat treatment is performed while supplying oxygen O ₂ . In the heat treatment, the n-type silicon substrate 1 from which the solid-phase diffusion source on the back surface 1B side in the stage of FIG. 2B is removed is put into a heat treatment furnace, and the temperature is increased and heated while switching the temperature and the film formation atmosphere. Lower the temperature. The atmosphere during heating is, after the n-type single crystal silicon substrate 1 is put in the furnace, heated in an atmosphere containing an inert gas such as nitrogen or argon in a temperature range of 800 ° C. to 1100 ° C. for an arbitrary time. And a step of heating in an atmosphere containing oxygen in a temperature range of 800 ° C. to 1100 ° C. for a time of 1 minute to 20 minutes or less.

  First, in an atmosphere containing, for example, an inert gas such as nitrogen or argon, a temperature T at which impurity diffusion from the BSG film 2 proceeds, for example, 800 ° C. to 1100 ° C. is reached, and a desired p-type diffusion layer 7 is formed. Form. After the formation of the p-type diffusion layer 7 is completed, oxygen is allowed to flow, so that the entire surface of the n-type single crystal silicon substrate 1 on which the p-type diffusion layer 7 is formed is oxidized as shown in FIG. A film 8 is formed.

The temperature profile of this heat treatment is shown by curve a in FIG. When the inside of the furnace is replaced with nitrogen and the furnace is preheated to become a nitrogen atmosphere and the temperature T = 900 ° C., the n-type single crystal silicon substrate 1 is charged into the heat treatment furnace at time t ₀₁ , and the time until time t ₀₂ is reached. t ₁ = 1 to 30 minutes. At the time t _02, supplying oxygen to the heat treatment furnace. While supplying oxygen, the temperature T is maintained for a time t ₂ = 1 minute to 20 minutes until the time t ₀₃ . In the oxidation step, the surface of the n-type single crystal silicon substrate 1 put in the heat treatment furnace is oxidized by oxygen contained in the atmosphere. The oxidation proceeds selectively on the back surface side not covered with the film because the light receiving surface side is covered with the BSG film 2 and the silicon oxide film 3. At time _t03 , supply of oxygen is stopped, nitrogen gas is supplied, and nitrogen substitution is performed.

After the above heating process, the temperature is lowered while supplying nitrogen, and then at time _t04 , the n-type single crystal silicon substrate 1 is taken out from the heat treatment furnace, and the back surface oxide film removing step S107 is performed. Accordingly, the silicon oxide film 8 on the back surface 1B side is removed. As shown in FIG. 3A, the rear surface 1B is exposed after the silicon oxide film 8 is removed. If the silicon oxide film 8 formed on the back surface 1B is thin, the subsequent impurity diffusion to the back surface 1B may be performed without performing removal.

Thereafter, impurity diffusion to the back surface 1B is performed as necessary. Here, the case of using a phosphorus diffusion step with POCl ₃ gas for forming the n-type diffusion layer as an example. In this process, the POCl ₃ gas is thermally decomposed on the entire surface of the n-type single crystal silicon substrate 1 to form a phosphorous silicate glass (PSG) film first, and this is used as a diffusion source and subsequently heated in the subsequent heating process. Penetrates or diffuses. Thus, in step S108 in which the back surface diffusion is performed in the POCl ₃ gas atmosphere, the phosphorus in the phosphorus diffusion POCl ₃ gas is quickly diffused to the exposed back surface 1B, and the light receiving surface 1A side on which the p-type diffusion layer 7 is formed. Since the silicon oxide film 8, the BSG film 2, and the silicon oxide film 3 serving as diffusion barriers are formed, the incorporation of phosphorus is prevented. At this time, the back surface 1B side is arranged so as to be directly exposed to the furnace atmosphere by superimposing two sheets by using an apparatus described later, and the PSG film is formed to a desired thickness. The

  That is, as shown in FIG. 3B, phosphorus diffusion is selectively performed on the back surface 1B, and the n-type diffusion layer 14 is formed on the back surface 1B.

  After the formation of the n-type diffusion layer 14, the BSG film 2, the silicon oxide film 3, and the silicon oxide film 8 functioning as a barrier are removed using, for example, 5 to 25% hydrofluoric acid aqueous solution. At this time, an oxide film formed by washing with water, generally called a natural oxide film, may be used as a passivation layer described later or a part thereof. Alternatively, for the same purpose, an oxide film obtained by cleaning with water containing ozone may be used.

  Subsequently, in the pn junction separation step S109, the p-type diffusion layer 7 and the n-type diffusion layer 14 are separated. Specifically, for example, end surface etching in which several tens to several hundreds of n-type single crystal silicon substrates 1 that have undergone the above steps are stacked and the side surfaces thereof are etched by plasma discharge, or side end portions of the substrate surface or back surface are used. It is desirable to perform laser separation or the like performed by irradiating and melting a laser near or on the side of the substrate.

  The substrate end face was cut or etched as described above, and as shown in FIG. 3C, the p-type diffusion layer 7 was provided on the light receiving surface 1A side, and the n-type diffusion layer 14 was provided on the back surface 1B side. A solar cell substrate is formed.

  Note that this separation step may be omitted depending on the state of separation, that is, the magnitude of leakage current, or the cell arrangement in the module that will be the final power generation product.

  Thereafter, in the back surface insulating film forming step S110, the back surface insulating film 15b made of a silicon nitride film is formed on the back surface 1B by using, for example, plasma CVD. Note that a passivation layer may be formed between the silicon nitride film and the n-type diffusion layer. In this case, the passivation layer is preferably a silicon oxide film, and in addition to general thermal oxidation, an oxide film obtained by washing with water or washing with ozone-containing water as described above may be used.

  Subsequently, in the antireflection film forming step S111, the light receiving surface antireflection film 15a is similarly formed on the light receiving surface 1A side by using, for example, a silicon nitride film using plasma CVD. Note that a passivation layer may be formed between the silicon nitride film and the n-type diffusion layer.

  In this case, the passivation layer is preferably a silicon oxide film, an aluminum oxide film, or a laminate of both. When a silicon oxide film is used for the passivation layer, an oxide film obtained by washing with water or washing with ozone-containing water as described above may be used in addition to general thermal oxidation. When an aluminum oxide film is used, it is formed, for example, by plasma CVD or ALD (Atomic Layer Deposition). In this case, since the fixed charge included at the time of film-forming has the effect of improving the passivation capability, it is more preferable.

  Note that the order of formation of the light-receiving surface antireflection film 15a, the back surface insulating film 15b, and both passivation layers is not necessarily limited to the above-described order, and the order other than the above may be appropriately selected and formed. good.

  Thereafter, as shown in FIG. 3D, in the electrode forming step S112, the light receiving surface electrode 16a and the back surface electrode 16b are formed on the light receiving surface 1A side and the back surface 1B side, respectively. As the electrode material, for example, copper, silver, aluminum, or a mixture thereof is used. For example, a paste made by mixing a metal powder of copper, silver, aluminum or a mixture thereof, glass, a ceramic component powder, and an organic solvent is formed into a pattern of a desired shape by, for example, screen printing, and then dried. And formed by firing. In this way, the solar cell is completed.

  As described above, according to the method of the first embodiment, even when the BSG film 2 and the silicon oxide film 3 of the solid phase diffusion source are formed, even if a product including boron is formed around the back surface. Since it is removed before heating, impurity diffusion to the back surface can be prevented by subsequent heating.

  Therefore, it is possible to obtain a solar cell with a long carrier life and high photoelectric conversion efficiency, which is prevented from mixing impurities other than the target impurity on the light receiving surface and the back surface, or contamination of contaminants that form the opposite conductivity type. .

  FIGS. 5A and 5B are cross-sectional views of the n-type single crystal silicon substrate 1 when a defective film formation portion is partially generated during the formation of the BSG film 2 and the silicon oxide film 3. FIG. 3 is a diagram corresponding to FIG. 2B and FIG. 2D in the above-described manufacturing process. As shown in FIG. 5A, the film formation failure portion 9 is divided into a silicon oxide film formation failure portion 9a, a BSG formation failure portion 9b, and a BSG film and silicon oxide film formation failure portion 9c. The defective film formation portion 9 forms the p-type diffusion layer formation failure portion 10 including the p-type diffusion layer shallowing portions 10a and 10b and the p-type diffusion layer non-forming portion 10c.

  Further, when removing the solid phase diffusion source on the back surface, the light receiving surface 1A may be etched to cause defects such as thinning of the solid phase diffusion source on the light receiving surface 1A side.

In the first embodiment, after the impurity diffusion step in step S106, an oxidation process is performed in an atmosphere containing oxygen O ₂ as shown in FIG. For this reason, a portion of the film formation failure portion 9 where the film is thinned so that impurity diffusion is not performed, for example, the light receiving surface 1A of the n-type single crystal silicon substrate 1 immediately below the BSG film and the silicon oxide film formation failure portion 9c. Oxygen reaches, and as shown in FIG. 5B, a silicon oxide film 8 is formed in the same manner as the back surface 1B. Since the silicon oxide film 8 functions as a barrier that prevents entry of contaminants from the furnace body or atmosphere, contamination of the light receiving surface 1A during heat treatment is prevented. That is, when oxygen is introduced after impurity diffusion, an oxide film is formed on the defective film portion 9 or a portion where no film is formed, for example, the back surface 1B, so that entry of contaminants can be prevented. In addition, the silicon oxide film 8 is formed by introducing oxygen after the impurity diffusion is performed at the location where the impurity diffusion is performed.

  When the n-type diffusion layer 14 is formed, the silicon oxide film 8 functions as a part of a barrier that prevents n-type impurities from entering the light receiving surface 1A. The thickness is desirably 5 nm or more and 10 nm or less. If it is less than 5 nm, the function as a barrier is poor in a later step, and if it is 10 nm or more, the increase in the barrier function works conversely, increasing the risk that the n-type diffusion layer on the back surface 1B side will not be well formed. In the impurity diffusion step of the n-type impurity to the back surface 1B side, the n-type impurity is prevented from diffusing on the light receiving surface 1A side, and the n-type diffusion layer is favorably formed on the back surface 1B side. The oxidation step performed after the diffusion step is important. By controlling the progress of oxidation and controlling the film thickness of the silicon oxide film 8 to 5 nm or more and 10 nm or less, diffusion leakage can be reduced and a highly efficient solar cell can be obtained.

  In addition, the silicon oxide film formed by this oxidation process is important not only for the film thickness but also for the film quality, but since it is implemented by introducing oxygen after heat treatment by diffusion exceeding 800 ° C., it has a dense film quality. It is a highly barrier film. For this reason, the back surface oxide film removal step S107 can be omitted by controlling the film thickness to 5 nm or more and 10 nm or less. That is, as it is, it is possible to improve the diffusibility of the n-type impurity to the back surface 1B side while preventing the n-type impurity from entering the light receiving surface 1A side. In the case where the silicon oxide film 8 on the back surface 1B is removed prior to the diffusion of the back surface 1B, the upper limit may be thicker than 10 nm.

  Note that the atmosphere containing oxygen is a mixture of oxygen and an inert gas typified by nitrogen or argon at a flow rate ratio of 10% to 100%. When oxygen is 10% or less, the oxidation rate on the surface of the n-type single crystal silicon substrate 1 is slow, so that it is difficult to obtain the effect. Unevenness of the oxide film on the surface can occur, which is not preferable. Although oxygen may be 100%, the oxidation rate is limited by the diffusion of oxygen into the n-type single crystal silicon substrate 1, and the oxidation rate increases as the oxygen flow rate ratio increases. It is necessary to limit the time to a short time. For this reason, it is preferable to heat in an atmosphere containing oxygen from a margin of 15% to avoid oxygen distribution unevenness in the furnace, for example, 40%.

  Note that the slice damage removing process, the texture forming process, and the cleaning process are examples used to describe the process of the first embodiment, and are not limited to these. A process may be used and is not limited to the above process. Similarly, the back surface n-type diffusion layer 14 forming step, the pn junction separation step, the light receiving surface antireflection film 15a and the back surface insulating film 15b forming step, and the light receiving surface electrode 16a and the back electrode 16b forming step. However, any process may be used and is not limited to the above process. In addition, the order from the step of forming the n-type diffusion layer 14 to the step of forming the electrode 16 may be appropriately changed as long as it functions as a solar cell, and is not limited to the order described.

  Further, for the sake of explanation, the n-type single crystal silicon substrate 1, the BSG film 2 as the solid phase diffusion source, and the n-type diffusion layer 14 by phosphorous diffusion on the back surface 1B are used. Absent. As long as it functions as a solar cell, another silicon-based crystal substrate such as a polycrystalline silicon substrate or silicon carbide may be used as the substrate, and a p-type substrate may be used as the conductive type. Further, the solid phase diffusion source may be one containing impurities that form an n-type diffusion layer such as phosphorus silicate glass (PSG). For the diffusion opposite to the solid phase diffusion source, an impurity forming a p-type diffusion layer such as boron may be used. As described above, for the diffusion layer formed on the light receiving surface and the back surface of the substrate as well, it is possible to appropriately select the p-type or n-type and the impurity element forming the diffusion layer.

  According to the method for manufacturing a solar cell of the present embodiment, oxygen is supplied to the diffusion furnace while maintaining the diffusion temperature for the last certain time in step S105 of performing the heat treatment in an inert gas atmosphere in the continuous heat treatment step in the furnace. By introducing it, oxidation can be carried out while promoting diffusion. In other words, the oxidation can be performed only by switching the gas supplied to the diffusion furnace, the silicon oxide film 8 can be formed without increasing the number of steps, and effectively acts as a diffusion barrier to the light receiving surface 1A side when the back surface of the silicon oxide film 8 is diffused. To do. Further, since the oxide film is formed by high-temperature oxidation, an oxide film with good film quality can be obtained. When removing the solid-phase diffusion source on the back surface, the light-receiving surface 1A is etched and a defect such as thinning of the solid-phase diffusion source on the light-receiving surface 1A side occurs. The barrier property on the 1A side is improved, and the introduction of impurities during back surface diffusion is prevented.

  As described above, a solar cell having a long carrier life and a high photoelectric conversion efficiency can be obtained by suppressing the mixing of impurities other than the intended impurity on the light receiving surface and the back surface, or the mixing of impurities that form the opposite conductivity type or contamination. Realized.

  In the solar cell formed as described above, the first conductivity type impurity concentration in the first conductivity type diffusion layer on the back surface 1B side is formed on the surface where the second conductivity type impurity is not formed. Since the concentration of the first conductivity type impurity is always higher than the concentration of the second conductivity type impurity in the entire region of the first conductivity type diffusion layer, the desired first conductivity type impurity is not affected by the second conductivity type impurity. Impurity concentration can be obtained. That is, since the reflected light incident from the back surface 1B side can effectively contribute to power generation, the characteristics of the double-sided light receiving solar cell can be improved.

Embodiment 2. FIG.
In the solar cell manufacturing method according to the second embodiment, a partial high-concentration diffusion layer is formed on either or both of the light-receiving surface side and the back surface side as compared with the solar cell manufacturing method shown in the first embodiment. To do. Since it is the same except for the oxide film removal step on the back side and the phosphorus diffusion step, detailed description is omitted as referring to the first embodiment.

  FIG. 6 is a flowchart showing a process from the heat treatment to the pn junction separation process in the solar cell manufacturing method according to the second embodiment. FIG. 7A and FIG. 7B are schematic views showing changes in the cross section of the n-type single crystal silicon substrate 1 during the n-type impurity diffusion step. Hereinafter, a description will be given with reference to FIGS. 6 and 7.

In the method for manufacturing the solar cell according to the second embodiment, after performing steps S104, 105, and 106, which are heat treatment steps for forming the p-type diffusion layer 7, as shown in FIG. In addition, a film forming step S108a on the back surface side of the solid phase diffusion source and a back surface diffusion step S108b which is a heat treatment step are performed. Here, a diffusion source 17 containing an n-type conductivity impurity at a high concentration, for example, 1 × 10 ²⁰ / cm ³ or more of phosphorus is formed on the silicon oxide film 8 on the back surface 1B. Thereafter, after the formation of the diffusion source 17, the n-type single crystal silicon substrate 1, which is the back surface diffusion step S108b, is subjected to heat treatment in a POCl ₃ gas atmosphere as in the back surface diffusion step S108 of the first embodiment. Impurity diffusion from the diffusion source 17 is performed at a temperature of 800 ° C. to 1000 ° C., for example.

Although the silicon oxide film 8 formed on the back surface 1B exists immediately under the diffusion source, its thickness is as thin as 5 to 10 nm, and the impurity concentration of the diffusion source 17 is high. It will not affect its formation. The diffusion source 17 in this portion is formed of a phosphorous silicate glass (PSG) film formed by thermal decomposition of POCl ₃ gas, and impurities are contained in the n-type single crystal silicon substrate 1 in contact with the diffusion source 17. Is diffused to form a high-concentration n-type diffusion layer 18. An n-type diffusion layer 20 having a lower concentration than the n-type diffusion layer 18 is formed in a region not covered with the diffusion source 17.

  Then, after the pn junction separation step S109, the back surface insulating film forming step S110, the antireflection film forming step S111, and the electrode forming step S112 shown in FIG. 1 are performed.

  On the other hand, the n-type single crystal silicon substrate 1 in a region other than directly under the diffusion source 17 is attached with impurities desorbed from the diffusion source 17 into the atmosphere, but its concentration or total amount is lower than the impurity concentration of the diffusion source 17 itself. It cannot pass through the oxide film formed on the surface of n-type single crystal silicon substrate 1.

  Therefore, according to the second embodiment, a structure having two levels of concentration can be formed in the diffusion layer. If the distribution of the two is appropriately performed, the region other than the region directly below the diffusion source 17 can be suppressed to a lower concentration, so that a more efficient solar cell is realized.

  As described above, the step of forming the solid phase diffusion source is selectively formed on the back surface 1B that is the second main surface, and the first conductivity type is formed by diffusion from the PSG film that is the solid phase diffusion source 17. This is a step of forming a certain n-type diffusion layer 20.

  Therefore, according to the second embodiment, the oxide film removing step on the back surface 1B can be removed from the manufacturing method, and n can be performed without affecting the silicon oxide film 8 formed on the n-type single crystal silicon substrate 1. The process up to the diffusion process of the type impurities can be completed.

In the step of forming the n-type diffusion layer which is the first conductivity type diffusion layer, impurities of 1 × 10 ²⁰ / cm ³ or more are applied to the back surface 1B which is the second main surface of the n-type single crystal silicon substrate 1. It forms a diffusion source containing. By this method, an impurity diffusion layer can be formed even if a silicon oxide film is present at a portion in contact with the diffusion source, and the silicon oxide film 8 on the back surface 1B of the n-type single crystal silicon substrate 1 is removed. The process can be omitted. In addition, since the diffusion from the diffusion source is performed while the entire surface of the n-type single crystal silicon substrate 1 that is a solar cell substrate is covered with the silicon oxide film 8, impurities released from the diffusion source into the atmosphere are n. Even if it adheres to the type single crystal silicon substrate 1, it does not diffuse inside the substrate.

  As described above, according to the method for manufacturing the solar cell according to the second embodiment, the step of removing the oxide film on the back surface is not required, so that the formation of a leak path in which p-type and n-type impurities are adjacent to each other can be prevented. A solar cell with excellent characteristics is realized.

  Note that FIG. 8 illustrates an example of a method for introducing a silicon substrate into a diffusion furnace used in a diffusion process in the heat treatment for the solar cell manufacturing method described in Embodiments 1 and 2. It is a figure which shows the injection | throwing-in method of the board | substrate to a heat processing furnace in order to perform the impurity diffusion to one side.

  The n-type single crystal silicon substrate 1 placed on the boat 201 is loaded into the heating furnace 200 to perform thermal diffusion. When forming either or both of the p-layer diffusion layer on the light-receiving surface side and the n-layer diffusion layer on the back surface side, two sheets of the n-type single crystal silicon substrate 1 are stacked on the heating furnace 200 as shown in FIG. Add them together as a set and heat. In the case of forming the p-layer diffusion layer, the back surface side is heated, and in the case of the n-layer diffusion layer formation, the light-receiving surface side is heated as a pair of two, respectively.

  Due to the characteristics of the shape of the mating surfaces, contact with the atmosphere during heating is limited, and the separation from the formed solid phase diffusion source or adhesion of the phosphorous silicate glass (PSG) film is suppressed. . Thereby, in the case of p layer diffusion heating, the penetration from the undesired impurities on the back surface and in the case of n layer diffusion heating can be further suppressed. Therefore, a high-quality solar cell with less leakage current is realized.

  In the present embodiment, after the heat treatment step, a part of the silicon oxide film as a protective film in the step of forming a conductive diffusion layer different from the formed diffusion layer is removed. Accordingly, it becomes possible to use impurity diffusion by a simple gas, and since the film remains except for the oxide film removal portion, it is possible to prevent the entry of impurities and the formation of a leak path.

  As described above, in the first and second embodiments, after forming a film containing an impurity serving as a solid phase diffusion source, the diffusion source on the back side is removed, and then the heat treatment is performed to remove the product on the back side. The manufacturing process for preventing the impurity diffusion from is shown. Specifically, in the case of the heat treatment, in the usual heat treatment, the treatment is performed using an inert gas such as nitrogen or argon, and then the heat treatment is performed in an atmosphere in which oxygen is introduced to perform a two-stage heat treatment. To implement. The supply of oxygen is performed after a heat treatment in an atmosphere not containing oxygen for diffusing impurities from the film of the solid phase diffusion source. In other words, the oxygen touched after furnace entry forms an oxide film as a diffusion barrier on the substrate backside with the product on the backside of the substrate. Impurity diffusion is performed from the film while the supply of oxygen is stopped. Impurities are diffused only on the surface. An oxide film is also formed on the film-forming surface of the solid-phase diffusion source by oxygen introduced at the end of the heat treatment, and a function as a barrier against another type of diffusion performed subsequently is added. By this method, impurities can be diffused only on the film formation surface.

  Note that the oxidation step after the diffusion step may be performed by introducing oxygen only for the last necessary time of the heat treatment step for diffusion, as described in Embodiment 1 with reference to FIG. In addition, oxygen may be introduced for a necessary time in the temperature lowering step after the heat treatment step. In addition, after the temperature of the diffusion furnace is once lowered to room temperature, an oxidation heat treatment step may be performed.

  In Embodiments 1 and 2, the temperature in the heat treatment step for impurity diffusion is determined by the type of impurity to be diffused and can be changed as appropriate. Also, the diffusion atmosphere can be a reducing atmosphere such as a hydrogen atmosphere in order to control the diffusion rate depending on the type of impurities, and can be adjusted as appropriate.

  In the first and second embodiments, an n-type single crystal silicon substrate is used as a semiconductor substrate. However, other crystal silicon substrates such as a p-type single crystal silicon substrate, p-type and n-type polycrystalline silicon substrates, and the like. Alternatively, it goes without saying that the present invention can also be applied to the formation of a diffusion layer using a compound semiconductor including a silicon compound such as silicon carbide SiC. The impurities of the first and second conductivity types are also determined in accordance with the conductivity type of the semiconductor substrate. The types of impurities are n-type impurities such as phosphorus, arsenic, and p-type impurities. Needless to say, the usual impurities other than gallium are applicable.

  Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope of the invention, and also included in the invention described in the claims and the equivalents thereof.

  1 n-type single crystal silicon substrate, 1A light-receiving surface, 1B back surface, 2 BSG film, 3 silicon oxide film, 4 boron-containing product, 5 silicon oxide-containing product, 7 p-type diffusion layer, 8 silicon oxide film, 9 composition Film failure portion, 9a Silicon oxide film formation failure portion, 9b BSG formation failure portion, 9c BSG film and silicon oxide film formation failure portion, 10 p-type diffusion layer formation failure portion, 10a, 10b p-type diffusion layer shallowening portion, 10c p-type diffusion layer non-formed portion, 14 n-type diffusion layer, 15a light-receiving surface antireflection film, 15b back surface insulating film, 16 electrodes, 16a light-receiving surface electrode, 16b back surface electrode, 17 diffusion source, 18 n-type diffusion layer, 200 heating Furnace, 201 boats.

However, in the method for manufacturing a solar cell disclosed in Patent Document 1, a phosphorous silicate glass (PSG) film or a boron silicate glass (BSG) film is formed on a substrate. It is subjected to a heat treatment for impurity diffusion in a nitrogen Kiri囲care after. For this reason, there is a problem in that impurities such as phosphorus or boron circulate on the back surface of the substrate during film formation, and impurities diffuse to the back surface simultaneously from the attached product, so that impurities are unintentionally mixed into the back surface. Impurity contamination leads to a reduction in the carrier life of the solar cell.

In order to solve the above-described problems and achieve the object, the present invention forms a solid phase diffusion source on the first main surface of the first conductivity type semiconductor substrate having the first and second main surfaces by the CVD method. And a heat treatment step of heating the semiconductor substrate, diffusing impurities of the second conductivity type from the solid phase diffusion source, and forming a diffusion layer of the second conductivity type. Prior to the step, a step of removing the solid phase diffusion source formed on the second main surface is included.

The solar cell according to the first embodiment uses an n-type single crystal silicon substrate 1 as a first conductivity type semiconductor substrate having a first main surface serving as a light receiving surface 1A and a second main surface serving as a back surface 1B. . The manufacturing method will be described with reference to FIGS. 1, 2A to 2D, FIGS. 3A to 3D, and FIG. First, damage layer removal step S101, and the contamination or damage that sometimes occurs on the surface of the wafer slicing the ingot into wafers, for example, an alkali was dissolved sodium hydroxide of less than 1 wt% or more 10 wt% solution, was immersed in removing After that, an antireflection structure is formed by adding an additive such as isopropyl alcohol or caprylic acid in an alkaline solution of 0.1% or more and less than 10% to the light receiving surface 1A of the n-type single crystal silicon substrate 1 and immersing it in the solution. The texture which is the unevenness | corrugation for obtaining is formed. The removal of slice contamination and damage and the formation of texture may be performed simultaneously or individually. The texture may be formed not only on the light receiving surface but also on the back surface. 2 and 3, the texture is not shown for easy understanding, and both the light receiving surface and the back surface are shown as flat surfaces.

FIG. 6 is a flowchart showing the process from the removal process of the solid phase diffusion source on the back surface to the separation process of the pn junction in the method for manufacturing the solar cell according to the second embodiment. FIG. 7A and FIG. 7B are schematic views showing changes in the cross section of the n-type single crystal silicon substrate 1 during the n-type impurity diffusion step. Hereinafter, a description will be given with reference to FIGS. 6 and 7.

The method of manufacturing a solar cell according to the second embodiment, after performing the step S104,105,106 is more punished Polytechnic for forming a p-type diffusion layer 7, sequentially shown in FIGS. 7 (a) As described above, the film forming step S108a on the back surface side of the solid phase diffusion source and the back surface diffusion step S108b which is a heat treatment step are performed. Here, a diffusion source 17 containing an n-type conductivity impurity at a high concentration, for example, 1 × 10 ²⁰ / cm ³ or more of phosphorus is formed on the silicon oxide film 8 on the back surface 1B. Thereafter, after the formation of the diffusion source 17, the n-type single crystal silicon substrate 1, which is the back surface diffusion step S108b, is subjected to heat treatment in a POCl ₃ gas atmosphere as in the back surface diffusion step S108 of the first embodiment. Impurity diffusion from the diffusion source 17 is performed at a temperature of 800 ° C. to 1000 ° C., for example.

Claims (7)

Forming a solid phase diffusion source on a first conductivity type semiconductor substrate having first and second main surfaces;
Heating the semiconductor substrate, diffusing impurities of the second conductivity type from the solid phase diffusion source, and forming a diffusion layer of the second conductivity type,
Prior to the heat treatment step,
A method for manufacturing a solar cell, comprising a step of removing the solid phase diffusion source formed on the second main surface.   2. The method of manufacturing a solar cell according to claim 1, further comprising an oxidation heat treatment step of forming an oxide film by continuously heating in an oxygen-containing atmosphere after the heat treatment step.   The method for manufacturing a solar cell according to claim 2, wherein the oxidation heat treatment step is a step of forming an oxide film having a thickness of 5 nm to 10 nm.   2. The heat treatment step is performed in a state in which the two semiconductor substrates are stacked, the first main surface is on the outer side, and the second main surface is on the mating surface side, and are stacked. 4. The method for producing a solar cell according to any one of 3 above.   5. The step of forming a diffusion layer of the second conductivity type includes a step of selectively forming the solid phase diffusion source on a part of the second main surface. 2. A method for producing a solar cell according to item 1.   The step of forming the diffusion layer of the second conductivity type is selectively performed on the second main surface through the oxide film while the oxide film formed in the oxidation heat treatment step remains on the second main surface. The method for producing a solar cell according to claim 1, further comprising a step of forming the solid phase diffusion source. A first conductivity type semiconductor substrate having first and second main surfaces;
An impurity diffusion layer of a second conductivity type formed on the first main surface;
An impurity diffusion layer of a first conductivity type formed on the second main surface,
The concentration of the first conductivity type impurity diffusion layer on the second main surface is higher than the concentration of the second conductivity type impurity diffusion layer in the entire region.

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